1. Field of the Invention
The present invention relates to an objective lens disposed at the tip portion of an endoscope, and particularly relates to an objective lens for an endoscope having a focusing mechanism.
2. Description of the Prior Art
Conventionally, a general objective lens for an endoscope comprises a first lens group with positive refractive power and a second lens group with negative refractive power, and has about an 8-100 mm viewing range on the object side.
In an endoscope employing such an objective lens, the viewing subject is located in the depth of field of the objective lens, and picture images are transmitted by a solid imaging element such as a CCD or an image guide fiber.
Moreover, in order to observe viewing subjects such as the affected parts as a whole, it is preferable to locate an objective lens apart from the viewing subjects, and to locate the objective lens close to the viewing subjects so as to observe the viewing subjects in detail. In particular, in recent medical treatments demanding highly precise observation and more accurate diagnosis, it is desirable to develop an objective lens for an endoscope which can provide preferable picture images over a wide viewing range.
However, in the above-described conventional objective lens for an endoscope, the depth of field is fixed because of a fixed diaphragm, and viewing subjects cannot be clearly observed beyond the depth of field.
There is also an objective lens for an endoscope having a focusing mechanism. However, since the objective lens consists of a first lens group having negative refractive power and a second lens group having positive refractive power, it is difficult to preferably correct various aberrations such as coma when a concave lens is too powerful in the first lens group with negative refractive power, which thus requires further improvements.